Polymer Tubing
The AP-600 Alternate Polymer extrusion system from Harrel can start with a tube made 100% from polymer "A" and change the composition to polymer "B". The transition will be tightly programmed. The outside and inside diameters of the tubing may be the same for the two polymers, or either or both may be changed independently.
Polymers "A" and "B" may be different durometers of the same polymer, or they may be entirely different. Polymer requirements are the same as for any coextrusion application in that the two polymers must be compatible.
The tubing resulting from the Alternate Polymer process has far higher structural integrity than a combination of two tubes joined together.
The Harrel AP-600 Alternate Power system does the alternate polymer tubing transfer from one polymer to another by means of precision melt pumps, with highly accurate servo drives. Meltpumps are capable of a degree of control over the polymer stream that cannot be achieved any other way. The output of a melt pump is proportional to the RPM (to 0.1%), and the melt pump is a shut-off device. When the melt pump stops, the flow of polymer stops.
To make tubing of constant dimensions but varying composition, the Alternate Polymer Tubetrol system controls these pumps so that the sum of the two speeds is always constant as one increases and the other decreases. The net throughput of the system can thus be held constant to within approximately 0.1% through the entire alternating polymer cycle. If the puller speed is held constant, a constant net throughput means that the cross sectional area of the product will also remain constant. The OD and the ID are then adjusted continually using the techniques of Harrel's Bump Tubing Tubetrol system, to correct dimensional changes due to the polymer change.
The dimensions of the tubing can be programmed to change to produce any other desired configuration, using Harrel Bump Tubing Tubetrol techniques.
A very high degree of control and automation has been built into the Alternate Polymer Tubetrol system. Two separate CP-873 Digipanel controllers are used. One handles overall system control, plus the control of the polymers, varying the proportions of each as programmed, while the other monitors and controls the dimensions of the overall tube at each point along its length, again in accordance with the specified target dimensions. The system can only be offered as a packaged, turnkey system. Plastic goes in one end, and a finished Alternate Polymer tube comes out the other.
All of the convenience features of the CP-873 DIGIPANEL controllers are also available in the Alternate Polymer Tubetrol system. These include full color schematic diagrams of the system, with out-of-tolerance values in red, the ability to "speak" any of four languages at the touch of a button, a trend plot facility that can display the history of any four product dimensions or extrusion parameters for the past hour, 11 "standby" heating modes, automatic start up and ramp up routines, starting routine menus, and unlimited recipe storage on standard 3 1/2" diskettes.
One of the advantages of the dual Digipanel control used in the Alternate Polymer Tubetrol system is that it permits a full range of Statistical Quality Control tools. Histograms, control charts, and SQC summaries are available using either the Harrel CP-603 single line SQC module, or the CC-620 Plant Wide Control and Management System.
The initial design of the Alternate Polymer system used a standard extrusion die, and this can still be done if desired. A new die features reduced inventory in the channels to further reduce polymer residence time in the die. It also uses the Harrel Spirodie construction to guard against spider lines, or weld lines in the final tube, where the multiple paths of the individual polymers join in the crosshead.
The Alternate Polymer system also uses a screw design to tolerate the extreme variations in back pressure that the extruder sees as the gear pump speeds change. This supplements the feedforward provisions included in the control system to bring the problem down to manageable levels.
The Alternate Polymer system pictured uses 1" geartruders for an output of 15 lbs/hr and is intended for small tubing. There are no size limitations. Alternating polymer pipe is an obvious application. The Alternate Polymer system is equally applicable to making profile, sheet, blown film, or any other extruded product.
Harrel Inc., 16 Fitch St., E. Norwalk, CT 06855. Tel: 203-866-2573; Fax: 203-866-1483.